White Fluorescent Organic Light-Emitting Diodes with 100% Power Conversion

Energy-efficient lighting sources are desired to provide another solution of carbon emission reduction. White organic light-emitting diodes are promising, because of theoretical internal quantum efficiencies for 100% electric-to-light conversion. However, pure organic fluorescent materials still face a challenge in harvesting triplet excitons for radiation. Herein, we report a white fluorescent organic light-emitting diode having an external quantum efficiency of 30.7% and a power efficiency of 120.2 lm W−1. In the single emissive layers, we use blue thermally activated delayed fluorescent emitters to sensitize a yellow fluorescent emitter. Transient photoluminescence and electroluminescence analyses suggest that a blue thermally activated delayed fluorescent molecule with ~100% reverse intersystem crossing efficiency and negligible triplet nonradiative rate constant completely converts triplet to singlet, suppressing triplet quenching by a yellow fluorescent emitter and ensuring 100% power conversion.

Their long-lived feature makes triplets inherently easy to be quenched by intermolecular interactions.Therefore, enhancing triplet-to-singlet conversion is a fundamental way to restrain quenching-induced efficiency reduction and roll-off [35][36][37].It is demonstrated that TADF molecules can be used as triplet sensitizers of traditional fluorescent (FL) emitters [38,39].The so-called "hyperfluorescence" diodes with emissive layers containing FL and TADF molecules can also achieve η EQE more than 20%.In these devices, singlet excitons converted from the first triplet excited state (T 1 ) of TADF sensitizers can be subsequently captured and utilized by FL emitters, through Förster resonance energy transfer (FRET).This process not only makes singlet-triplet equilibrium become more favorable to singlet formation but also transforms relatively labile charge transfer excitons of TADF molecules into stable Frenkel excitons of FL emitters.It is beneficial to comprehensively mitigate nonradiative deactivation of excitons [3].However, triplet energy transfer from TADF sensitizers to the dark T 1 state of FL emitters should be thoroughly avoided.Dexter energy transfer (DET) of triplets is based on short-distance charge exchange (within 1 nm).Thus, low doping concentration of FL emitters is believed to be necessary and sufficient to maintain a long enough average distance between TADF and FL molecules and thereby prevent triplet DET (Fig. 1A).However, at present, hyperfluorescence OLEDs hardly achieved η EQE reaching the state-of-the-art value of TADF diodes (30%).It means that a part of excitons was still wasted during the energy transfer process [40,41].
Charge transfer excitons of TADF molecules are much easier to long-range delocalize, compared to Frenkel excitons.Besides DET, another feature of triplet excitons must be noted: triplet diffusion distance (≥10 nm) is at least one order of magnitude larger than singlet diffusion distance (≤1 nm) [42] (Fig. 1A).In this case, triplet diffusion actually provides another channel for triplet capture by FL emitters.Triplet DET and triplet diffusion are respectively dominant at relatively high and low doping concentration.The energy level relationship between TADF sensitizers and FL emitters shows that it is difficult to completely eradicate triplet migration between their T 1 states, since effective FRET should be based on a suitable concentration of FL emitters (Fig. 1B).Therefore, an "ideal" situation is triplet-involved processes confined on TADF molecules, which requires the following: (a) the TADF sensitizer completely converts triplet to singlet, and (b) before conversion, triplet nonradiation is effectively suppressed.As a proof of concept, in this contribution, we demonstrate a white hyperfluorescence system featuring triplet-free exciton allocation.Three blue TADF emitters named ptBCzPO 2 TPTZ, 2CzPN, and DMAC-DPS are respectively used as blue-emitting sensitizers to fabricate hyperfluorescence white OLEDs with a conventional yellow FL emitter TBRb (Fig. 1C).These 3 molecules have similar molecular polarities, thus excluding the influence of dipole-dipole interactions between TADF and FL emitters.In the 4,6-bis(diphenylphosphoryl)dibenzofuran (DBFDPO) matrix with a doping concentration of 40% for weight percentage, the RISC efficiency (ϕ RISC ) and triplet nonradiative rate constant (k T nr ) of ptBCzPO 2 TPTZ are 97% and 2.15 × 10 3 s −1 , respectively.In contrast, ϕ RISC values of 2CzPN and DMAC-DPS (~80%) are much lower, while the k T nr value of the 2CzPN-based film (2.24 × 10 3 s −1 ) is equal to that of ptBCzPO 2 TPTZ, but is only one-fifth of that of the DMAC-DPS-based film (1.26 × 10 4 s −1 ).The orthogonal correlation of ϕ RISC and k T nr values for these molecules establishes the basis for figuring out exciton utilization and quenching processes and the relative structure-property relationships.Blue emissions from ptBCzPO 2 TPTZ, 2CzPN, and DMAC-DPS are complementary to the yellow emission of TBRb, and simultaneously overlap with the π→π* absorption band of TBRb in a large range of 400 to 550 nm, leading to efficient FRET (Fig. 1D).Compared to 2CzPN and DMAC-DPS, ptBCzPO 2 TPTZ with ~100% ϕ RISC and negligible k T nr effectively mitigates triplet quenching by TBRb.As a result, warm-white OLED of DBFDPO:40% ptBCzPO 2 TPTZ:0.1% TBRb achieved a stateof-the-art η EQE of up to 30.7%, corresponding to 100% η IQE , and a record-high power efficiency (η PE ) of 120.2 lm W −1 .

Steady-state photophysical properties
The photoluminescence (PL) quantum yield (η PL ) of the DBFDPO:40% ptBCzPO 2 TPTZ film reaches 94%, which is higher than ~70% of 2CzPN-and DMAC-DPS-doped films (Fig. 2A to C).More importantly, for DBFDPO:x% ptBCzPO 2 TPTZ:y% TBRb dually doped films, at x = 40, a slight codoping of 0.1% TBRb largely increases η PL to 99%, reflecting nearly unitary radiative efficiency and completely suppressed non ra diative transitions.However, further increasing TBRb concentration gradually decreases η PL .Similarly, when x ≤ 40, the η PL of DBFDPO:x% ptBCzPO 2 TPTZ:0.1% TBRb films gradually increases and reaches the maximum at x = 40 (Fig. S1).Further increasing x also induces η PL decrease.The η PL inflexion is a result of a delicate balance between exciton radiation and quenching.In contrast, for DBFDPO:40% 2CzPN:y% TBRb films, η PL decreases to 47% even at y = 0.1, and further halved to ~20% at y ≥ 0.2.The quenching effect of TBRb is the worst for DBFDPO:40% DMAC-DPS:y% TBRb films, whose η PL is as low as ~10% when y ≥ 0.1.Notably, ptBCzPO 2 TPTZ-based films reveal gradually reduced η PL inversely proportional to x and y, reflecting a linear dependence of triplet quenching on ptBCzPO 2 TPTZ-TBRb distance.On the contrary, the η PL of 2CzPN-and DMAC-DPS-based films sharply decreases to the minimum values at quite low y, especially for the latter, indicating the dominance of relatively distance-insensitive triplet diffusion in emission quenching.It shows that η PL variation is directly related to ϕ RISC of these blue TADF emitters.
At extremely low TBRb concentration (y = 0.1), the yellow emission intensities are independent of blue TADF emitters (Fig. 2A to C and Fig. S2).The ~100% η PL of the DBFDPO:40% ptBCzPO 2 TPTZ:0.1% TBRb film is a combined result of 100% FRET efficiency, RISC enhancement, and triplet quenching suppression.In contrast, 2CzPN-and DMAC-DPS-based films show slightly changed PL spectra with sharply decreased η PL , reflecting exciton allocation-induced triplet nonradiation.Since triplet diffusion rather than triplet DET is predominant at y = 0.1, it suggests that triplet diffusion-induced quenching is negligible for ptBCzPO 2 TPTZ-based films, but significant in 2CzPN-and DMAC-DPS-based films.Furthermore, along with y increasing, yellow emissions of DMAC-DPS-based films rapidly increase, which are largely stronger than those of 2CzPNbased films, while ptBCzPO 2 TPTZ-based films display the weakest yellow emissions.Since FRET is a dipole-dipole resonancebased long-range energy transfer, these 3 blue TADF emitters are comparable in the FRET process.The large PL spectra difference of the dually doped films is mainly caused by intermolecular interaction-based exciton/charge exchange, namely, singlet DET.The combined analysis of η PL and PL variations suggests that η PL and yellow intensity are simultaneously related to the intermolecular interaction intensity in the dually doped films.Symmetrically bar-shaped DMAC-DPS has the strongest intermolecular interactions.At the same time, the highest occupied molecule orbital and lowest unoccupied molecule orbital energy levels of TBRb are respectively shallower and deeper than those of DMAC-DPS, leading to direct charge/ exciton capture by TBRb (Fig. 3A).Consequently, singlet and triplet diffusions in DMAC-DPS-based films are the strongest, giving rise to the most marked yellow emissions and the lowest η PL .On the contrary, the asymmetric and sphere-like configuration of 2CzPN somewhat reduces intermolecular interactions and limits singlet/triplet diffusions and DET, rendering the weaker yellow emissions but the higher η PL values of 2CzPN-based films, which is similar to our previously reported all-TADF systems [29].In comparison, ptBCzPO 2 TPTZ with large steric hindrance further restrains intermolecular interactions and singlet/triplet diffusion in its films.Triplet DET becomes the main channel of emission quenching, which is also limited but still appreciable at high x and y.Moreover, for ptBCzPO 2 TPTZ-based films, compared to DBFDPO, another conventional host, bis{2-[di(phenyl)phosphino]-phenyl}ether oxide (DPEPO), with a larger steric hindrance further decreases yellow emission intensity and mitigates η PL reduction at high y (Fig. S3).On the contrary, without host, η PL reduction is accelerated (Fig. S4).

Time-resolved emission properties
Transient emission spectra (TRES) show that compared to blue-emitting 2CzPN and DMAC-DPS films, 0.1% TBRb induces marked delayed fluorescence (DF) quenching (Fig. 2D), and DF intensities and lifetimes are approximately independent of TBRb concentration (0.1% to 1.0%) (Fig. S5).This result suggests that DF quenching in 2CzPN and DMAC-DPS films is primarily due to triplet diffusion, which is consistent with their η PL variations.In contrast, the smallest k T nr makes the triplet state of ptBCzPO 2 TPTZ much less sensitive to TBRb doping; therefore, TRES contours of DBFDPO:x% ptBCzPO 2 TPTZ:y% TBRb films (x = 40%; y = 0 and 0.1) are nearly identical, despite the gradual DF reduction at y > 0.1 (Fig. S6).In the DPEPO matrix, the DF variation of ptBCzPO 2 TPTZ is similar (Fig. S7).At y = 0.1, when x ≤ 40, DF intensities and lifetimes remain sta ble, but prompt fluorescence (PF) intensity is directly proportional to x.It means that in this x range, triplet quenching can be effectively suppressed, while increasing x reduces ptBCzPO 2 TPTZ-TBRb distance, thus enhancing FRET.At an x range of 40 to 100, different to slightly changed PF, DF intensity and lifetime sharply decrease from x = 40 to 60 and further gradually decline until x = 100.Actually, at x = 100, the PF and DF properties of the binary films are independent on y.Therefore, when ptBCzPO 2 TPTZ becomes the majority (x > 50), DET via direct interactions between ptBCzPO 2 TPTZ and TBRb is still inevitable, rendering triplet quenching.Nevertheless, the critical x% reaching 40% still demonstrates that ~100% ϕ RISC and the largely reduced k T nr of ptBCzPO 2 TPTZ indeed effectively alleviate triplet quenching by TBRb, which establishes the feasibility of balancing FRET and triplet exclusion for optimal exciton allocation.
PF time decay curves of singly doped blue-emitting films are nearly identical to those of blue PF emissions from 0.1% TBRb codoped films (Fig. 2D and Fig. S8A).Time decay curves of yellow PF and DF are consistent with those of the corresponding blue PF and DF, respectively.PF lifetimes of TBRb in dually doped films are markedly larger than those of the DBFDPO:5% TBRb film (Fig. S8B).Therefore, yellow emissions of TBRb in these films are mainly ascribed to FRET from blue TADF emitters.However, 0.1% TBRb significantly shortens blue DF lifetimes of 2CzPN/DMAC-DPS-based films.In contrast, for DBFDPO:x% ptBCzPO 2 TPTZ:y% TBRb films, at x = 40, blue DF time decay curves overlap for y = 0 and 0.1, owing to effectively suppressed triplet diffusion (Fig. S9).Further increasing y of ptBCzPO 2 TPTZ-based films hardly changes blue PF lifetimes, but gradually reduces blue DF lifetimes, reflecting intensified DET-induced triplet quenching.On the other hand, at y = 0.1, increasing x from 10 to 40 induces elongated blue PF lifetimes of ptBCzPO 2 TPTZ, but does not change its blue DF lifetimes (Fig. S10).In this x range, direct excitation of ptBCzPO 2 TPTZ's S 1 state is gradually enhanced, but triplet quenching can still be effectively controlled.However, further increasing x from 40 to 100, blue PF lifetime becomes stable, but blue DF lifetime gradually declines, reflecting marked triplet quenching.At x = 100, increasing y from 0.1 to 1.0 can still shorten blue DF lifetime, but the variation is much smaller than at x = 40.These results further suggest that for DBFDPO:x% ptBCzPO 2 TPTZ:y% TBRb films, x = 40 is the critical point of balancing FRET and triplet quenching, and only a considerable x or y increase leads to serious triplet quenching, indicating the predominance of triplet DET in the quenching process.
Compared to the DBFDPO:40% ptBCzPO 2 TPTZ film, the ϕ RISC of the DBFDPO:40% ptBCzPO 2 TPTZ:0.1% TBRb film is further improved to 99%, and in particular, its k T nr largely decreases by one order of magnitude to 3.0 × 10 2 s −1 (Fig. 2E and Table S1).Simultaneously, its singlet radiative rate constant (k S r ) significantly increases by one-third to 1.6 × 10 7 s −1 .Consequently, 0.1% TBRb facilitates RISC and singlet radiation and mitigates triplet quenching in ptBCzPO 2 TPTZ-based films.In this case, triplet excitons can be completely converted to radiative singlets by ptBCzPO 2 TPTZ and then respectively allocated to itself and TBRb, resulting in 100% triplet harvesting.In contrast, 0.1% TBRb induces sharp ϕ RISC decreases of 8% and 12% for 2CzPN-and DMAC-DPS-based films, respectively.Furthermore, the k T nr of the 2CzPN-based film is doubled, but its k S r is halved, in addition to 13% increased k S nr .Although the k T nr and k S nr of the DMAC-DPS-based film are reduced by 0.1% TBRb doping, its k S r decreased by 70-fold to 1.0 × 10 5 s −1 , which was 1 / 10 of its k S nr .Therefore, TBRb doping hinders RISC and aggravates quenching effects in 2CzPN-and DMAC-DPS-based films.These results show that in these dually doped films, compared to ~70% ϕ RISC of 2CzPN and DMAC-DPS, ptBCzPO 2 TPTZ with ~100% ϕ RISC and small k T nr establishes a basis for TBRb to largely enhance radiative transitions and reduce nonradiation.Notably, the TADF parameters of the ptBCzPO 2 TPTZ:0.1% TBRb film are comparable to those of the DBFDPO:40% 2CzPN:0.1% TBRb film.It suggests that despite effectively limited triplet diffusion, triplet DET to FL dopants can still cause serious quenching of TADF sensitizers.

EL performance
White OLEDs were fabricated by vacuum evaporation, following the simple 3-layer architecture (Fig. 3A).Single emissive layers DBFDPO:x% blue TADF emitter:y% TBRb were employed, in addition to using N,N′-dicarbazolyl-3,5-benzene and 2,4,6-Tris(4-(diphenylphosphoryl)phenyl)-1,3,5-triazine as hole and electron transporting layers, respectively.Concentrations of x and y were carefully explored to determine the optimal parameters (Figs.S11 to S16 and Table S2).Simultaneously, in accord with PL spectra, increasing x and y enhanced yellow emissions in EL spectra, as a result of FRET improvement.Compared to 2CzPN and DMAC-DPS with stronger intermolecular interactions, ptBCzPO 2 TPTZ-based devices displayed dual-peak complementary white emissions with largely higher color purities (Fig. 3B).It is noted that at y = 0.1, when x ≥ 60, the EL spectra of ptBCzPO 2 TPTZ-based devices almost overlapped.It indicates that at x ≥ 60, the FRET process tended to be stable, and triplet DET became dominant, leading to unchanged exciton allocation ratios for blue and yellow emissions.In contrast, at x = 40, along with y increasing from 0.1 to 1.0, EL correlated color temperature changed from 6,225 K (y = 0.1), 4,914 K (y = 0.5), to 3,921 K (y = 1.0) (Fig. 3C and Fig. S17), which are respec tively close to correlated color temperature values of standard illuminants D65 (6,500 K, artificial daylight), D50 (5,000 K, simulated sunlight), and A (2,856 K, incandescent light).
It is shown that at x = 40, ptBCzPO 2 TPTZ-based devices realized the best performances (Figs.S11 and S14).In the y range of 0.1 to 1.0, besides luminescence beyond 30,000 cd m −2 , the driving voltages at 1, 100, and 1,000 cd m −2 were as low as ~2.5, ~3.3, and ~4.1 V, respectively (Fig. 3D and Fig. S14), which are the lowest values reported so far among white hyperfluorescence OLEDs [37,43] and also comparable to the best results from all kinds of white EL devices [44] (Table S3).Moreover, the driving voltages were roughly inversely proportional to x. Obviously, at high doping concentrations, ptBCzPO 2 TPTZ with ambipolar feature made significant contributions to carrier injection and transportation.2CzPN-and DMAC-DPS-based devices revealed similar situations (Figs.S12 and S13).Nevertheless, ptBCzPO 2 TPTZ-based host-free devices (x = 100) displayed slightly larger driving voltages than those of DBFDPO hosted analogs.Moreover, the incorporation of DPEPO with worse electrical properties increased driving voltages.Therefore, the electrical performance of the host matrix is also considerable for balancing carrier flux, while quenching suppression by the host matrix improves luminance at specific current density (J), thereby, in turn, reducing driving voltages.Furthermore, the incorporation of TBRb largely increased the maximum luminance, since FRET to TBRb facilitates RISC and restrains triplet concentration quenching.
Considering a singlet ratio of 25% and an outcoupling ratio of 20% to 30% for indium tin oxide (ITO) glass, an η EQE of ~5% corresponds to 75% to 100% singlet utilization.Therefore, for a DMAC-DPS-based device with a maximum η EQE of ~5%, DF was almost negligible.In contrast, ptBCzPO 2 TPTZ-and 2CzPNbased devices exhibited ~100% and 50% singlet and triplet utilization, respectively.Furthermore, EL TRES shows that the DF intensity of the DBFDPO:40% ptBCzPO 2 TPTZ:0.1% TBRbbased device was markedly higher than that of the 2CzPN-based analog (Fig. 4A).The blue DF EL lifetime of ptBCzPO 2 TPTZbased devices was largely longer than that of the 2CzPN-based analog, manifesting the reduced triplet quenching on ptB-CzPO 2 TPTZ.The situation of yellow DF EL lifetime was similar.It is noted that within 150 μs, the yellow intensity of the 2CzPNbased device was stronger than that of the ptBCzPO 2 TPTZbased analog.However, after 150 μs, the yellow DF intensity of the former was, in turn, smaller than that of the latter.This variation is consistent with faster triplet DET and simultaneously stronger triplet quenching in the former.
Singlet (N S ) and triplet (N T ) exciton densities of DBFDPO:40% Blue TADF emitters:0.1% TBRb devices can be calculated as a function of current density (J)(18) (Fig. 4B): in which k ISC and k RISC are rate constants of ISC and RISC, and d and e are recombination zone thickness and electron charge, respectively.k STA and k TTA are rate constants of singlet-triplet and triplet-triplet annihilations, respectively.The N S and N T values were in the ranges of 10 11 to 10 17 and 10 14 to 10 19 , respectively.It is shown that the N S and N T of DBFDPO:40% ptBCzPO 2 TPTZ:0.1% TBRb are almost the same as those of DBFDPO:40% ptBCzPO 2 TPTZ, manifesting the predominant roles of ptBCzPO 2 TPTZ in exciton conversion and utilization.Notably, k STA and k TTA values of ptBCzPO 2 TPTZ-based devices reaching the levels of 10 −10 and 10 −15 s −1 are comparable to those of FL OLEDs [45] but markedly higher than those of common TADF diodes [18] (Fig. 4C).k STA and k TTA are directly proportional to the oscillator strength of an exciton utilizer; therefore, it demonstrates that despite its charge transfer featured excited state, ptBCzPO 2 TPTZ (η PL = 94%) is equal to locally excited TBRb (η PL = 84%) in terms of exciton radiation, resulting in the balance of blue and yellow efficiencies for the state-of-theart white OLEDs.In contrast to 2CzPN-and DMAC-DPS-based analogs, ptBCzPO 2 TPTZ significantly reduces N S by about 30-and 400-fold, and N T by 4-and 5-fold in its white OLEDs, (1) (2) respectively, and increases k STA and k TTA by more than 10-and 90-fold, respectively.Thus, blue TADF emitters are dominant in exciton kinetic processes of these hyperfluorescence white diodes.Undoubtedly, compared to 2CzPN and DMAC-DPS, the superiority of ptBCzPO 2 TPTZ in radiation facilitation and nonradiation suppression is the primary reason for the stateof-the-art performance of its white FL OLED.

Discussion
We have demonstrated new record EL efficiencies of 30.7% for η EQE and 120.2 lm W −1 for η PE with a single-emissive layer white FL OLED, indicating the potential of pure organic OLEDs for large-scale daily lighting application.It is shown that 100% exciton utilization can be readily realized by hyperfluorescence white-emitting systems, when triplet DET and diffusion-induced quenching can be effectively suppressed.The fundamental solution is to reduce triplet concentration in the device, which requires TADF sensitizers to have ~100% ϕ RISC and low enough k T nr .Simultaneously, TADF sensitizers with a large steric hindrance are advantageous in reducing triplet diffusion to FL emitters.It is noted that FRET from eligible TADF materials, like ptBCzPO 2 TPTZ herein, to FL emitter can further improve RISC and singlet radiation and alleviate triplet nonradiation.These results provide new insights into optimizing exciton allocation and utilization in TADF-based systems.We obtained a power efficiency of 65.3 lm W −1 at 1,000 cd m −2 , which can be doubled to over 130 lm W −1 by additional outcoupling enhancement, thereby surpassing the current benchmarks of daily lightings (100 lm W −1 for white LED and 70 lm W −1 for FL tube).For large-scale applications, the lifetime issue of blue TADF emitters should be solved in the future.Nevertheless, our results suggest that pure organic white OLEDs hold the promise of satisfying all demands for ideal lighting sources, including high efficiency, low cost, ecofriendliness, and so on.

Transient emission measurement
The films for measurement were prepared by vacuum evaporation.PL TRES were measured by an Edinburgh FLS 1000 fluorescence spectrophotometer using a time-correlated single photon counting method with a nanosecond and a microsecond pulsed light source for 100 ps to 10 s lifetime measurement, a synchronization photomultiplier for signal collection, and a multichannel scaling mode of the PCS900 fast counter PC plug-in card for data processing.

Fabrication and characterization of OLEDs
The ITO substrate was cleaned with detergent and deionized water, dried in an oven at 120 °C for 4 h, treated with oxygen plasma for 3 min, and then transferred to a deposition chamber.Devices were fabricated by evaporating each layer onto the ITO substrate sequentially at a pressure below 1 × 10 −6 Torr.A MoO 3 layer was first deposited on the ITO substrate at 0.1 nm s −1 .The deposition rates of organic layers were 0.1 to 0.3 nm s −1 .Then, a 1-nm layer of LiF was deposited at 0.1 nm s −1 .Finally, a 100-nm-thick layer of Al was deposited at 0.6 nm s −1 as the cathode.The devices were then transferred to a glovebox and encapsulated with hot melt glue.The emission areas of the devices were 0.09 cm 2 .EL spectra were measured with a PR655 spectra colorimeter.Current-density-voltage and brightnessvoltage curves of the devices were measured using a Keithley 4200 source meter and a calibrated silicon photodiode.All the measurements were carried out in atmosphere.

Fig. 3 .
Fig. 3. OLED performance.(A) Device structure and chemical structures of used materials.(B) Electroluminescence (EL) spectra at 1,000 nits.(C) Commission Internationale de l'Eclairage (CIE) coordinates and correlated color temperature (CCT) values under different doping concentrations at 1,000 nits.(D) Luminance-voltage-current density (J) relationships.(E) Efficiency-luminance correlations.CE, PE, and EQE refer to current efficiency, power efficiency, and external quantum efficiency, respectively.PE and EQE values at 100 and 1,000 cd m −2 were highlighted with arrows.

Fig. 4 .
Fig. 4. EL kinetics of white hyperfluorescence OLEDs.(A) Time-resolved EL emission spectra (top) and EL time decays at blue and yellow peak wavelengths (bottom) of devices with emissive layers of DBFDPO:40% blue TADF emitter:0.1%TBRb at 1,000 nits.(B) Relationship between singlet (N S , round symbols) and triplet (N T , triangle symbols) exciton densities and J in white OLEDs and blue diodes of DBFDPO:40% ptBCzPO 2 TPTZ.(C) Comparison on rate coefficients of singlet-triplet (STA, k STA ) and triplet-triplet annihilations (TTA, k TTA ) for the devices.a.u., arbitrary units.